Shossing, or shossin' comes from a combination of the words, shit, shower, and toss or tossing.
To be one who is a shosser, or guilty of shossing they must first have shat into their own hand whilst in the shower, and then tossed said feces into the toilet.

'remember that girl from the other night'
-"yeah, howd that go? you toss her salad?"
'yeah well the next morning she tried shossing for the first time, IN MY BATHROOM, that bitch missed the bowl and just climbed out my bathroom window.'
-"i hope it was solid."